The present invention relates to an electronic device for measuring the blood pressure of a patient and a method of operation thereof, and more particularly relates to such a device for blood pressure measurement and such a method of operation thereof, of the type adapted to continuous measurement, in which the squeezing of the patient's arm by a cuff incorporated in said device, and the inevitable attendant discomfort and errors due to blood congestion engendered by such squeezing, are minimized.
Generally, in the continuous measurement type of blood pressure measurement device, conventionally a cuff is fitted over the arm of a patient and is inflated by a pressure pump, thereafter being deflated progressively. During these processes, the pressure inside the cuff is measured and a pulse wave signal is obtained therefrom. Typically, in order to provide for continuous blood pressure measurement, the cycles of inflation and deflation of the cuff must be performed repeatedly, and thus the peaks of cuff inflation pressure at which the patient's arm is most squeezed are repeated. However, the problem has existed of pain caused to the patient at such times of high cuff inflation pressure, and furthermore there is a possibility for the patient's arm to become congested with blood at these times, which can adversely affect the blood pressure measurement process and cause erroneous blood pressure measurements.
It is conventionally known that these peaks of cuff inflation pressure are only required for the measurement of the systolic blood pressure while on the other hand the average blood pressure and the diastolic blood pressure are measured at other points in the inflation and deflation cycle for the cuff, at which the pressure in the cuff is relatively lower. Accordingly, in order to overcome such problems and others, there was conceived, and there was specified in Japanese Patent Application Serial No. 59-49750 (1984), which has been laid open to the public as Japanese Patent Laying Open Publication Ser. No. 60-193440 (1985), which it is not intended hereby to admit as prior art to the present patient application except to the extent in any case required by applicable law, and which was applied for and is beneficially owned by the same entity as the entity to whom the present patent application is assigned or is the subject of a duty of assignment, an electronic blood pressure measurement device, comprising: (a) a cuff for being fitted around the arm of a patient; (b) a means for selectively pressurizing said cuff with fluid, so as to squeeze said arm of said patient; (c) a means for selectively draining said fluid from said cuff either at a relatively rapid rate or at a relatively slow rate; (d) a means for sensing the pressure of said fluid in said cuff and for producing an output signal representative thereof; (e) a means for receiving said pressure signal from said cuff pressure sensing means and for generating therefrom a signal representative of the pulse wave of the patient; (f) a first blood pressure measuring means for determining for the patient a systolic blood pressure, an average blood pressure, and a diastolic blood pressure according to said pulse wave signal, during the course of pressurization and gradual evacuation at said relatively slow rate of said cuff; (g) a means for computing the value of a constant as being the ratio of the difference of said systolic blood pressure and said average blood pressure to the difference of said average blood pressure and said diastolic blood pressure, all said blood pressures being as determined by said first blood pressure measuring means; (h) a second blood pressure measuring means for determining for the patient an average blood pressure and a diastolic blood pressure according to said pulse wave signal, during the course of gradual evacuation said relatively slow rate of said cuff; and: (i) a means for computing the value of a systolic blood pressure, according to said average blood pressure and said diastolic blood pressure as determined by said second blood pressure measuring means, and according to the value of said constant as computed by said computing means therefor. Further, this electronic blood pressure measurement device included a control means, which first performed the steps of operating said first blood pressure measuring means so as to determine for the patient a systolic blood pressure, an avverage blood pressure, and a diastolic blood pressure according to the current pulse wave signal, and of operating said constant value computing means so as to determine the value of said constant, and thereafter repeatedly performed the steps of operating said second blood pressure measuring means for determining for the patient an average blood pressure and a diastolic blood pressure according to said pulse wave signal, and of operating said means for computing the value of a systolic blood pressure according to said average blood pressure and said diastolic blood pressure as determined by said second blood pressure measuring means and according to the value of said constant as first determined. Further, at a certain fixed time interval, i.e. after the lapse of a determinate time period, the control means returned to the above described first stage of operating said first blood pressure measuring means and of operating said constant value computing means, so as thereby to determine anew, i.e. to refresh, the value of said constant. There was further disclosed a corresponding method of operation of said electronic blood pressure measurement device. According to this electronic blood pressure measurement device and this method of operation thereof, the squeezing of the patient's arm was reduced, because only in the first above described stage of operation, i.e. during measurement of the systolic blood pressure as well as of the average blood pressure and of the diastolic blood pressure, was it necessary for the pressure in the cuff to be raise to a very high level; during the second above described stage of operation, i.e. during measurement only of the average blood pressure and of the diastolic blood pressure, only light squeezing of the patient's arm was required. Thereby the patient's arm was only required to be relatively severely squeezed, once in every said determinate time period. Accordingly, relatively severe squeezing of the patient's arm, and pain and congestion of the patient's arm due to blood accumulation therein caused thereby, which might unnecessarily disturb the blood pressure measurement process, were reduced.
However, this type of electronic blood pressure measurement device, and the method of operation thereof, although they achieved a certain improvement over prior arts in that the squeezing of the patient's arm was much reduced, were not perfect. In detail, they were subject to the problem that, although from time to time the value of the constant absolutely was needed to be updated by the control device returning to the first stage of operation as described above, because the blood pressure wave form of any individual patient necessarily must change over time and accordingly errors tended to accumulate, nevertheless the most ideal time points for such refreshment of the constant did not necessarily occur at a fixed time interval.
If for example in fact the blood pressure wave form of the patient had not changed significantly, then it was undesirable to return to the first stage of operation, because such a first stage of operation performed relatively severe squeezing of the patient's arm, and accordingly caused pain and some congestion of the patient's arm due to blood accumulation therein, which unnecessarily disturbed the blood pressure measurement process. On the other hand, if in fact the blood pressure wave form of the patient were to change relatively quickly, then it might become desirable to return to the first stage of operation for redetermination of the constant before the complete elapsing of the determinate time period, because otherwise errors in blood pressure measurement might exceed the acceptable levels. The speed of alteration of the blood pressure wave form of the patient cannot be accurately predicted to be a constant in advance, because of variation of the measurement conditions and because of individual differences between patients. Accordingly, the above described electronic blood pressure measurement device, and the method of operation thereof, were subject to the twin problems of either liability to unduly high measurement errors, or of inflating the cuff to a high pressure level more often than actually necessary.